The following will describe the vacuum-processing device and the control method therefor according to the present invention in a case where they are applied to a vacuum soldering device (referred to also as “vacuum reflow device”). Conventionally, voids generated in hot blast reflow processing under a normal atmospheric pressure have been regarded as a problem in reflow mounting steps for large-current elements such as mounting of power devices, power modules and the like, and a technique of decreasing the generation of voids has been demanded.
FIGS. 14A and 14B are schematic views each showing a hot blast reflow example according to a conventional example. Cream solder 8 shown in FIG. 14A is applied onto a pad electrode 4 in a substrate 5. The cream solder 8 has appropriate viscosity due to addition of flux to solder powder, and is applied onto the pad electrode 4 in the substrate 5 through a mask by means of a screen printer.
In this conventional hot blast reflow, the cream solder 8 is subjected to hot blast reflow processing, and, when the solder has been brought in a molten state, voids 2 are generated in the molten solder. Also when the molten solder (molten solder 7) is cooled and solidified, these voids 2 disadvantageously remain as they are in the solder.
Regarding the generation of voids, a state where the cream solder 8 is applied onto the pad electrode 4 in the substrate 5 and subjected to the hot blast reflow processing under an atmospheric pressure with no electronic component being mounted will now be schematically explained with reference to FIGS. 14A and 14B. The solder 3 shown in FIG. 14B indicates a state such that, after the hot blast reflow processing of the cream solder 8 shown in FIG. 14A, the molten solder 7 has been cooled and solidified in a spherical shape by surface tension. The outlined white circle shapes in this figure represent the portions of the voids 2, which have unintentionally been produced in the molten solder 7 and still remain in the solder 3 even after cooling and solidification thereof. The voids 2 impair heat conduction effect and cause deterioration of heat exhaust in power devices and the like.
Concerning the reduction in generation of voids as described above, Patent Document 1 discloses a soldering device (vacuum reflow device) having the vacuum evacuation function. This soldering device includes an exhaust valve, a vacuum pump and a processing tank, and is configured so that a substrate is imported into the processing tank and, in a molten state of solder on a pad electrode in the substrate, the exhaust valve is opened to drive the evacuation pump, thereby once evacuating the processing tank. Such a vacuum state allows the voids remained in the solder during solder melting to be removed by virtue of the defoaming and/or deaerating effect.